While running a 2012 SQL Server Integration Services package, the package ended up failing with the following message:

Error: 0xC002F304 at Send Mail Task, Send Mail Task: An error occurred with the following error message: "Mailbox unavailable. The server response was: 5.7.1 Client does not have permissions to send as this sender".

The quickest and easiest fix? Take a look at your SMTP connection in the Connection Manager. Uncheck the Windows Authentication check box like so: